SAW devices having interdigital transducers have been described in David P. Morgan's treatise titled Surface Wave Devices for Signal Processing, Elsevier Science Publishing Co., N.Y., (1985), ISBN 0-444-42511-X, incorporated herein by reference in its entirety. Generally, a surface acoustic wave (SAW) device, such as a delay line, comprises a transmitting (launching) and a receiving transducer placed on the surface of a piezoelectric substrate. The transmitting transducer uses voltage variations derived from input radio frequency electro-magnetic signals to induce mechanical flexures of the atoms along the surface of a piezoelectric substrate, thereby creating surface waves having acoustic properties. The substrate supporting the surface waves is generally made of a piezoelectric material such as quartz SiO.sub.2, lithium niobate LiNb0.sub.3, lithium tantalate LiTaO.sub.3, bismuth germanium oxide Bi.sub.12 GeO.sub.20 or other similar substances. The surface acoustic waves thus created travel along the surface of the substrate at speeds ranging from about 3159 meters per second for Quartz and 3992 meters per second for lithium niobate to as slow as 1830 meters/second for bismuth germanium oxide. The voltage induced surface acoustic waves travel away from the transmitting transducer along the surface of the substrate until they reach the receiving transducer. The receiving transducer performs the inverse function of the transmitting transducer, that is, it converts the arriving surface waves on the substrate surface to a voltage resembling the one originally impressed on the transmitting transducer.
Ideally, the function of above described SAW structure is to recreate a delayed, but undistorted copy at the receiving transducer of the original voltage waveform convolved with the designed impulse response of the SAW device. However, because of reflections arising from the interaction of various structures associated with the transmitting and receiving transducers, a faithful, undistorted convolution of the original waveform with the impulse response is not achieved at the receiving transducer.
There are two types of reflections referenced in the present invention that interfere with the reaction of the desired output signal. A first type of reflection is due to the physical presence and dimensional extent of multiple fingers of interdigitated transducer structures interacting with the surface of the piezoelectric substrate. The presence of the transducer structure and its edges causes undesired, interfering reflections related to the geometric arrangement, size, and conductivity of the transmitting and receiving transducers. This type of interfering reflection is caused by the discontinuity on the surface of the piezoelectric substrate represented by the physical presence of the transducer structures. These reflections emanate from the interaction of the acoustic wave as it encounters the dissimilar material of the many interdigitated fingers on the surface of the substrate. These reflections add coherently from each of the fingers to either reinforce or reduce at various times certain portions of the desired signal. Furthermore, these reflections exist even if the transducer producing them presents a zero electrical impedance to the electrical current generated by the incoming acoustic wave, i.e. is shorted. This type of reflection will be referred to as a "mechanical-electrical loading" reflection due to essentially its mechanical nature. This reflection is designated as P.sub.11 by Morgan in Electronics Letters 26, Eq 5, pg 1200 (1990) and by Hartmann and Abbott in the Ultrasonics Symposium Proceedings, Eq 1, pg 80 (1989). A well known technique for minimizing this type of reflection depends on "split fingers", or "double electrode transducer" technique as discussed, for example, in U.S. Pat. Nos. 4,353,046 and 4,162,465, and is well known in the art.
The maximum operating frequency of a split finger device is is generally only half that of SAW devices having non-split finger construction assuming the same lithographic limitations on the physical etching of the interdigital transmitter and receiver structures. This is because the split finger design requires the presence of two fingers (as compared to one), with attendant decrease in finger width and increase in lithographic complexity. It is therefore desirable to have non-split finger construction.
A second type of reflection is generated from the interaction of the receiving transducer output voltage with the surface of the substrate as induced by the voltage of the output waveform. This type of reflection emanates from the receiving transducer. Subsequently, the reflection travels from the output transducer to the input transducer, and is reflected back from the structure of the input transducer. This reflection is distinguished from the first type because it is not generated by the mechanical presence of the transducer structure, but by electric fields associated with the output voltage that has to be created by the transducer to operate with a useful electrical source or load impedance of typically 75 or 50 .OMEGA.. This reflection will generally be referred to as the "regenerated" load related reflection. This reflection is given by the second term of equation 5 of Morgan's Electronics Letters 26, pg 1200 (1990), referenced above. The quantites referred in this term can be computed using analysis methods familiar in the current art. One manifestation of this type of reflection is the triple transit distortion well known in the art, and is minimized in the prior art by the use of multiphase matching networks as described, for example, in U.S. Pat. No. 3,686,518. Such a multiphase matching structure however has the disadvantage that it requires three phase networks as well as SAW interdigital structures that cannot be constructed in a single metallization step, needing air gap crossovers, which add substantially to SAW manufacturing costs.
The prior art has attempted to concurrently control both types of reflections to minimize the total reflection (S.sub.11 in equation 5 of Morgan's Electronics Letters referenced above) as, for example, in U.S. Pat. No. 4,353,046. Here, generally, "mechanical-electrical loading" reflections are reduced by split finger structures, while the "regenerated" reflections are reduced by selective loading of the split finger structures with metal during various metallization steps. The problem of frequency limitation due to the split finger structure as well as manufacturing costs associated with the metallization of the split finger structure impact negatively the utility of such a SAW device.
Above referenced teachings indicate that when simultaneously controlling the distortions due to the two types of reflections described above, other parameters, such as, metallized surface area and manufacturing costs, are suboptimal as compared to the present invention.
At this point a distinction is introduced between "generator/receivers", which herein mean that portion of a SAW device that converts electrical energy into an acoustic wave and a "transducer" which means a combination of a "generator/receiver" and a reflector structure used to suppress reflections from the generator/receiver. The transducer structure can convert from electrical to acoustic energy as well as from acoustic to electrical energy.